Transposons generate allelic diversity in bacteria, fungi, plants, and animals. New alleles transmitted to progeny provide diversity upon which natural selection acts, hence transposons are one of the underlying mechanisms of evolution. In addition, on the time scale of a single organism, transposons can create diversity of gene expression within the body. The generation of immunoglobin diversity in mammals, including humans, depends on the excision of transposons that divide the antibody genes into several non-functional parts; only after transposon excisions are the gene parts joined to make a functional allele. Joining is an error-prone process that generates millions of combinations of antibody-producing alleles. This diversity of alleles is crucial to our survival,-because we can use a small number of inherited genes to dynamically generate a wide spectrum of alleles to defend against pathogens. The principal aim of this study is to understand a developmental switch that results in a different transpositional outcome in gametes containing heritable allelic diversity compared to strictly somatic diversity created in the body. The MuDR/Mu (Mutator) transposons of maize move by a "cut and paste" mechanism in somatic tissues, very late in development resulting in a wide spectrum of new alleles in a fine mosaic in the plant body. This parallels the antibody gene example. In contrast, maize gametes contain new mutations caused by insertion but no excision alleles that generate diversity in a particular gene. The molecular and biochemical mechanisms programming the highly regulated timing of events and switch in transpositional outcome will be elucidated by analyzing proteins (MURA transposases and MURB helper proteins) encoded by the regulatory transposon MuDR. Host factors that regulate MuDR/Mu activities will also be characterized, because transposable element activities are an evolved nartnershiD of the transDoson and its host.